This invention relates to methods and apparatus for use in casting, particularly for use in casting large, iron alloy articles such as cylinder heads and cylinder blocks for internal combustion engines.
Traditional casting methods generally employ a xe2x80x9cgreen sandxe2x80x9d mold which forms the external surfaces of the cast object and the passageways into which the molten iron alloy is poured for direction into the mold cavity. A green sand mold is a mixture of sand, clay and water that has been pressure formed into the mold element. Green sand molds have sufficient thickness so that they provide sufficient structural integrity to contain the molten metal during casting and thereby form the exterior walls of the casting. The structural integrity of the green sand molds, however, is not completely satisfactory and the green sand can easily yield to the pressure that may be exerted by the hands of a workman.
For example, in casting a cylinder head, a green sand mold is provided with a cavity and preformed cavity portions to position and hold core elements that form the exhaust gas, air intake, and coolant passageways and other internal passageways in the cast cylinder head.
The coolant passages are frequently formed with two core elements to permit the interlacing of a one-piece core element forming the plurality of air intake passageways to the cylinders and a one-piece core forming the plurality of exhaust gas passageways from the plurality of cylinders. In such methods, a first element of the coolant core is placed in the green sand mold and core elements forming the passageways for the air intakes, and for the cylinder exhausts are then placed in the green sand mold and the second element of the coolant core is joined with the first element of the coolant core, frequently with the use of adhesive. This method entails substantial labor costs and opportunities for unreliable castings. Where adhesive is used, it is necessary that the workman apply the adhesive correctly so that it will reliably maintain the coolant jacket core elements together during casting. It is also necessary that the workman reliably assemble the two elements of the coolant jacket core during manufacture, and assemble the separate core elements in the green sand mold without damaging the interfacing portions of the green sand mold that reliably position the core elements one with respect to the other. This manufacturing method provides an opportunity for the green sand of the mold to be deformed by a workman in assembly of the core elements within the green sand mold, and an opportunity for a lack of reliability in maintaining a reliable location of the plurality of core elements one to the other. The result is that there is no assurance that the thickness of the internal walls of the cylinder head will be reliably maintained during the manufacture, and there is a substantial risk that unreliable castings will result.
This method was improved by the method set forth in U.S. Pat. No. 5,119,881 issued Jun. 9, 1992. This improved method permits a plurality of inter-engaging one-piece core elements to form an integral core assembly, with interlaced passage-forming portions that are reliably positioned and maintained in position to form a cylinder head with reliable wall thickness and an opportunity to decrease the metal content. In this improved method, a core assembly includes for example a one-piece coolant jacket core, a one-piece exhaust core and a one-piece air intake core, all reliably positioned and held together in an integral core assembly that eliminates the more unreliable core element assembly by manufacturing personnel in the green sand mold. In this improved manufacturing method, the integral core assembly was placed in the green sand mold as a whole prior to pouring the molten iron alloy into the green sand mold.
In such casting, the core elements that form the internal passageways of the cylinder head are formed with a high-grade xe2x80x9ccore sandxe2x80x9d mixed with a curing resin so that core elements may be formed by compressing the core sand-curing agent mixture, and curing the resin while compressed to form core elements that have sufficient structural integrity to withstand handling and the forces imposed against their outer surfaces by the molten metal that is poured into the mold cavity. The core sand resin is selected to degrade at temperatures on the order of 300 to 400 degrees Fahrenheit so that the core sand may be removed from the interior of the cylinder head after the molten iron alloy has solidified.
Because of the cost of the core sand, it is desirable that the sand be recovered for further use after it has been removed from the casting. Recovery of the green sand used in the mold is also desirable; however, the large quantities of the green sand-clay mixture can be degraded sufficiently during the casting process that they cannot be economically recycled and must be hauled away from the foundry and dumped. Since the production of such castings is frequently hundreds of thousands of cylinder heads per year, the cost of handling and disposing of the green sand residue of the casting process imposes a significant unproductive cost in the operation of the foundry. In addition, the core sand frequently becomes mixed with the green sand to such an extent that the core sand cannot be reused in the casting process.
The invention eliminates the use of green sand by replacing green sand molds with a xe2x80x9ccore sandxe2x80x9d assembly that can provide, during casting, both the internal and external surfaces of the cylinder head or other casting, such as a cylinder block. In the invention, a mold is formed from the same core sand that is used to form the core elements defining the internal passageways of the casting. After the mold and core elements, both of which are formed from core sand, are assembled, they are placed in a carrier with sides that hold the assembled mold and core elements together during pouring of the molten iron alloy into the mold-core assembly and the cooling period during which the molten iron alloy solidifies to form the casting. The carrier for the mold-core assembly may take several forms, including, for example, an insulative shell cast from refractory lining materials used, for example, in lining a smelting furnace. The refractory shell may have sufficient thickness to support the core sand mold-core assembly during pouring operations, or may comprise a thinner walled refractory shell carried within a supporting metal framework. Such refractory shell elements may be used for a multiplicity of casting operations before they need to be discarded or repaired. Preferably, however, the carrier can comprise thin, replaceable metal walls supported by a surrounding supportive structure that is sufficiently xe2x80x9copenxe2x80x9d to expose outside surfaces of the thin, replaceable walls to the ambient atmosphere for cooling.
In the process of the invention, a plurality of mold carriers are provided and a plurality of core sand mold-core assemblies are provided. The mold-core assemblies comprise core sand mold-forming elements and core sand core-forming elements. The mold-core assemblies are loaded, one after another, into the mold carriers and are transported to a pouring station where the core sand mold-core assemblies are filled with molten metal. The poured mold-core assemblies and carriers are then allowed to cool until the castings are formed and are transferred after the cooling period to an unloading station where the carriers are inverted, the castings are retrieved and the core sand is removed from the interior cavities of the castings. The castings are then ready for inspection and further machining operations, and the core sand is recovered and returned to provide a further plurality of core sand elements, either mold elements or core elements or both.
In the invention, the use of green sand is eliminated by replacing the green sand molds with a combination of reusable, mold-core assembly carriers and mold elements and core elements that are formed by core sand. By eliminating the use of green sand, the cost of the green sand and its clay binders, the problems associated with mixing of the green sand and core sand and their respective binders, and the environmental costs of disposing of the excess green sand are eliminated.